The present invention relates generally to magnetic bearings which may be used, for example, to bear the rotor of a blood pump which is implanted into the human body to assist the heart. Magnetic bearings are ideally suited for a blood pump since they allow the rotor to be suspended relative to the stator and therefore allow free flow of blood without obstructions so that it does not stagnate and thus coagulate and allow the blood to flow along a path large enough that individual blood cells are not damaged due to shear.
Bearings for blood pumps as well as other pumps and motors must not only bear the rotor radially (journal bearing) but must also bear the rotor axially (thrust bearing). Reliable axial control of the rotor is particularly important in blood pumps since too much movement of the rotor axially may narrow a blood pathway thereby restricting blood passage so much that individual blood cells may become damaged due to shear and the blood may coagulate. In addition, it is important that the power consumption in blood pumps be low since the heat resulting from high power consumption may damage or destroy blood cells, and low power consumption is also desirable to reduce the operating costs.
Patents (in addition to those cited hereinafter) which may be of interest in the development of magnetic bearings include U.S. Pat. Nos. 5,084,643; 5,133,527; 5,202,824; 5,666,014; 5,175,457; and 5,521,448, which above patents are hereby incorporated herein by reference.
An example of radial and axial magnetic bearings for blood pumps is found in U.S. Pat. No. 6,201,329 which is assigned to the assignee of the present invention and which is hereby incorporated herein by reference. This application discloses a blood pump wherein permanent magnetic rings are provided in attraction on opposite sides of each of two axially spaced radial gaps to levitate the rotor, and an actively controlled magnetic means is provided across radial gaps to bear thrust.
While the above blood pump bearings are considered to work well, it is nevertheless considered desirable to simplify the magnetic bearing arrangement so as to provide a more compact blood pump as well as to reduce manufacturing cost. It is also considered desirable to reduce the bearing operating cost.
U.S. Pat. Nos. 4,944,748; 5,078,741; and 5,385,581 to Bramm et al disclose a magnetically suspended and rotated rotor having an axially polarized cylindrical permanent magnet at each end of the rotor. An axially polarized permanent magnet ring is provided at each end of the stator to magnetically interact therewith respectively to levitate the rotor. The stator magnet rings are offset axially outwardly of the rotor magnets respectively. An Electromagnet exerts a control force on the rotor magnets to keep the impeller at the null position, in the absence of additional static axial forces on the impeller. The electromagnet receives feed-back of rotor position (see col. 14, last paragraph, of the ""581 patent) to maintain the null position. As seen in FIG. 4 of the ""581 patent, it appears that one pole of the rotor magnet is used to levitate the rotor and the electromagnet flux interacts with the other pole to act as a thrust bearing. It is believed that this does not allow an arrangement of magnets to achieve a desired radial and angular stiffness to suitably support the rotor, with the result that the specific gravity of the fluid determines the rotor design (see the paragraph which spans cols. 26 and 27 of the ""581 patent) so that the rotor may not be able to levitate and spin without the presence of fluid.
When additional static axial forces are applied to the impeller of the above pump, as detected by the electronic circuitry, a different equilibrium position is thereafter maintained, instead of the null or previous equilibrium position (see the paragraph which spans cols. 11 and 12 of the ""581 patent). As discussed in the paragraph which spans cols. 22 and 23 of the ""581 patent, if an additional axial force is exerted on the impeller, this is detected by signals, provided to a difference amplifier by the position sensors, having a direct current component which is said to arise from a persistent small shift in the impeller position as opposed to random variations in impeller position about the null position. Thus, a new equilibrium position is established in response to these direct current component signals. Not only is such a system which uses displacement as feedback for establishing a new equilibrium position complex but it is also unreliable (prone to error) since there may be instances in which it may be difficult for the circuitry to xe2x80x9ctellxe2x80x9d whether there is an outside axial force or just instability causing a change in impeller position.
It is accordingly an object of the present invention to provide a simplified, compact, and reliable magnet assembly for the radial and thrust bearings for a rotor.
It is a further object of the present invention to provide low power consumption for such a magnet assembly.
In order to provide a simplified, compact, and reliable magnet assembly for the radial and thrust bearings for a rotor, in accordance with the present invention, first and second axially spaced combinations are provided each including at least one permanent magnet disposed on each of the rotor and stator and polarized to levitate the rotor, and there is further provided an electrically energizable coil for modulating flux between the respective stator and rotor magnets, and the rotor magnets are offset axially both inwardly or both outwardly of the stator magnets respectively.
In order to provide low power consumption for such a magnet assembly, in accordance with a preferred embodiment of the present invention, an electrical circuit responsive to feed-back of electrical energy to at least one of the coils is provided for comparing thereof with a reference electrical energy and integrating the differences therebetween to provide a signal to modify a reference position of the rotor, whereby to attain a zero force balance position wherein the current to the coils may be reduced to near zero.
The above and other objects, features, and advantages of the present invention will be apparent in the following detailed description of the preferred embodiment thereof when read in conjunction with the accompanying drawings wherein the same reference numerals denote the same or similar parts throughout the several views.